1. Field of the Invention
The present invention relates to a method for treating a waste water by employing oxidation to purify it. To be more specific, it relates to a method for treating waste water containing an organic compound having two or more carbon atoms and/or a nitrogen compound by oxidation where the waste water is subjected to oxidation and/or decomposition, and then treating the resultant liquid with a reverse osmosis membrane having high salt rejection rate where the liquid is separated into a liquid which contains a substance to be oxidized (herein after may be referred to as xe2x80x98oxidizable substancexe2x80x99) such as an organic substance and does not permeate through the reverse osmosis membrane, and a liquid which contains almost no oxidizable substance and permeates through the reverse osmosis membrane.
2. Description of the Related Art
As one of methods for treating waste water, oxidation has conventionally been suggested. However, the oxidation is not capable of sufficiently oxidizing and decomposing organic substance and nitrogen compound if they are hard-to-decompose. In the wet-oxidation, organic substances and nitrogen compounds in the waste water is oxidized and/or decomposed. In an attempt to achieve more excellent purification of waste water, the following processes have therefore been attempted: the waste water is subjected to wet-oxidation and the resultant liquid is concentrated by being treated with a reverse osmosis membrane, and then, the concentrated liquid is subjected to wet-oxidation again. For example, Japanese Unexamined Patent Publication No. 1-262993 suggests such a method. Specifically, in this prior art method, the waste water is subjected to wet-oxidation in a reaction tower to suppress the generation of Nox-N, and the resultant liquid is treated with a reverse osmosis membrane to be separated into a liquid which does not permeate through the membrane and a liquid which permeates through the membrane. In the liquid which does not permeate through the membrane, the acid component is concentrated. The liquid containing the concentrated acid components (i.e. the liquid which does not permeate through the reverse osmosis membrane) is mixed with the waste water again so that the pH of the waste water is adjusted to 7, and the mixture is oxidized. By use of the reverse osmosis membrane, the acid components having pH of 1 to 3 are satisfactorily concentrated and remain in the liquid which does not permeate through the membrane; however, it has no ability of eliminating the acetic acid, and almost all the amount thereof passes through the reverse osmosis membrane. As a result, the liquid which permeates through the reverse osmosis membrane contains a oxidizable substance such as acetic acid, and waste water is not purified to a sufficient level.
The article titled xe2x80x9cthe development of waste water recycling technique by wet-oxidation using a catalystxe2x80x9d (xe2x80x9cDistilling techniquexe2x80x9d, Vol. 16, No. 3, page 13 to 24, 1990) describes a method in which waste water is subjected to wet-oxidation, and then processed with a reverse osmosis membrane of polyether type or polyvinyl alcohol type. The reverse osmosis membranes of polyethylene type such as polyethylene oxide type and polyethylene imine type, cellulose acetate type, polyvinyl alcohol type, and polyethel type have high ability of extracting (i.e. removing) acid components having a molecular weight of 100 or larger; however, they exhibit poor ability of eliminating acid components having a molecular weight of less than 100. It is impossible to sufficiently eliminate organic acids such as acetic acid having low molecular weight. In order to eliminate acetic acid, it is necessary to subject the liquid which permeates through the reverse osmosis membrane to another purification step such as methane fermentation.
The present invention has been accomplished to solve the above problems. Accordingly, it is an object of the invention to provide a method for treating waste water in which the waste water is subjected to oxidation, and the resultant liquid is treated with a reverse osmosis membrane to be further separated into a liquid containing a oxidizable substance such as organic substance, and a liquid containing almost no oxidizable substance.
The present invention provides a method for treating a waste water, comprising the steps of: oxidizing a waste water containing an organic compound having two or more carbon atoms and/or a nitrogen compound; separating the oxidized waste water into a permeated liquid having almost no oxidizable substance and an impermeated liquid having oxidizable substance such as organic acid and/or ammonia through a reverse osmosis membrane having high salt rejection rate.
Also, the present invention provides a method for treating the waste water, comprising the step of: separating the waste water into a permeated liquid and an impermeated liquid through the reverse osmosis membrane having high salt rejection rate; oxidizing all or a part of the impermeated liquid.
Further more, the present invention provides a method for treating the waste water, comprising the steps of: adjusting pH of the waste water to pH 4 or more; and treating the waste water through the reverse osmosis membrane having high salt rejection rate.